The present invention relates to a pneumatic tire for automobiles having improved characteristics of static discharge, and more particularly to a pneumatic tire including a tread having an improved electric conductivity.
In recent years, it is increasingly demanded to reduce a fuel cost for automobiles. Since tire performances, particularly the rolling resistance of a rubber material constituting a tread of tires, have a great influence on the fuel cost, development of rubber compositions for tire treads having a low rolling resistance has been made.
Rubber compositions incorporated with a reduced amount of carbon black or rubber compositions incorporated with an inorganic filler such as silica are proposed as rubber compositions having a low rolling resistance. For instance, rubber compositions for low fuel cost tires containing silica are disclosed in European Patent No. 0501227, Japanese Patent Publication Kokai No. 5-271447 and Japanese Patent Publication Kokoku No. 5-80503.
At one time, as disclosed in Japanese Patent Publication Kokai No. 61-122003, since a metal was mainly used in a car body and static electricity generating between a tire tread and the ground was a problem rather than the static electricity generating from the car body, the problem was solved, in order to reduce noise of radio, by raising the electrical resistance of tires to more than 200 Mxcexa9 so as not to cause the static electricity of tires to flow to the car body. However, plastic materials are widely used in recent cars for lightening, and, static electricity accumulated in cars increases, so that generation of static electricity from the car body increases. Moreover, if a rubber composition containing silica is used in the tread of tires, the electrical resistance of such tires is at least 2000 Mxcexa9 and far exceeds the level of the above-mentioned prior art. How to discharge the static electricity accumulated in cars is important for such cars.
Diene rubbers have been generally used in treads of tires. Since the diene rubbers themselves are poor in electric conductivity, the conductivity has been raised by incorporating carbon black having a conductivity into rubber compositions. Consequently, if it is contemplated to reduce the rolling resistance, namely to reduce fuel cost, by incorporating an inorganic filler such as silica or by decreasing the amount of carbon black to be incorporated, the conductivity of tread rubber compositions is markedly decreased due to the incorporation of silica which is poor in conductivity or due to decrease of the amount of carbon black incorporated.
The static electricity accumulated in a car body flows from a wheel rim to bead portions of the tire, then to side walls and reaches a tread a portion of the tire. If the rubber material of the tread is poor in conductivity, the static electricity cannot flow to the ground and is accumulated in the tire and car body. Therefore, if low conductive tires are attached to a car, noise of radio, a spark and the like generate due to static electricity accumulated in the car. In particular, sparking is very dangerous for cars using a low boiling hydrocarbon fuel such as gasoline.
It is an object of the present invention to provide a pneumatic tire which can satisfy demands contrary to each other, namely reduction of rolling resistance and impartment of electric conductivity.
A further object of the present invention is to provide a pneumatic tire including a tread having excellent rolling characteristics and improved static discharge characteristics.
Another object of the present invention is to provide a tread having a thin conductive rubber layer on its surface.
Still another object of the present invention is to provide a static dischargeable pneumatic tire having a thin conductive rubber layer on the surface of its tread.
These and other objects of the present invention will become apparent from the description hereinafter.
It has been found that even if the tread rubber itself is poor in electric conductivity, the conductivity of the tread of tires can be improved without substantial adverse effect on performances by providing a thin film having a good conductivity on the tread surface.
In accordance with the present invention, there is provided a pneumatic tire comprising a tread and a conductive thin film provided on the surface of said tread continuously in the circumferential direction of the tire, wherein said conductive thin film has a thickness of not more than 0.5 mm and a volume resistivity of not more than 105 xcexa9xc2x7cm, and said tread is made from a rubber composition comprising a diene rubber and 40 to 100 parts by weight of silica having a BET specific surface area of 100 to 300 m2/g per 100 parts by weight of said diene rubber, the content of said silica being at least 50% by weight based on the whole filler incorporated in said composition.
The term xe2x80x9csurface of treadxe2x80x9d or xe2x80x9ctread surfacexe2x80x9d as used herein includes the surface of the groove portion formed in the tread.
The conductive thin film is applicable to various tread rubbers. Even if a tread rubber is poor in conductivity, the problem in accumulation of static electricity can be solved by forming the conductive thin film on the tread surface. The tread prepared from the rubber composition as defined above has excellent rolling characteristics and, therefore, the pneumatic tire according to the present invention can achieve a low fuel cost and a high conductivity. The conductive thin film provided on the tread surface disappears at the ground-contacting surface by abrasion after running in a while and the tread rubber appears at the surface, while the conductive thin film remaining at the groove portion and shoulder portion of the tread still serves to impart the conductivity to the tread. Therefore, even if a rubber having a high rolling resistance is used in the conductive thin film, it does not lead to increase of fuel cost.
The conductive thin film can be prepared from a rubber composition containing at least 10 parts by weight of carbon black per 100 parts by weight of a rubber.